Heat transfer characteristics of a two-phase thermosyphon heat exchanger

In this paper, a special design for a two-phase thermosyphon heat exchanger is proposed. This design features an evaporator tube bundle consisting of smooth, corrugated or porous coated tubes. The prototype heat exchanger consists of two horizontal cylindrical vessels connected by two risers and a downcomer. Tube bundles placed in the lower and upper cylinder function as an evaporator and a condenser. The operation of a two-phase thermosyphon is determined primarily by the evaporatorʹs performance. Therefore, an experimental investigation was conducted to determine the effects of the evaporator tube pitch (1.7d and 2.0d), the liquid head and fluid type on heat transfer in this two-phase thermosyphon heat exchanger. The investigation concerned six prototype heat exchangers operating in a heat flux range of 5–70 kW/m2. As working fluids, distilled water, methanol and refrigerant R-141b were utilised. The tested two-phase thermosyphon heat exchanger operates in a vacuum, and therefore the working liquids boiled in temperatures ranging from 24 °C to 62 °C. The obtained results indicate that the two-phase thermosyphon heat exchanger performs more effectively with an evaporator bundle comprising of porous coated tubes than with corrugated or smooth tubes. The evaporation heat transfer coefficient is strongly dependent on the liquid level above the top tube row (5 mm, 15 mm and 20 mm).